In some cases, this can be beneficial if it helps to structure the first book...in others, it's a totally different book that has nothing to do with the one you're currently writing and it drives you nuts
If you ask me again when I'm going to finish, I'm going to stomp on your throat. I'm writing a book, not a grocery list. Now back off. <<< I swear, every time I'm at a family function, someone asks when I'll finish.